1. Field of the Invention
The present invention relates to a method for sorting paperboard blanks wherein a continuous paperboard web is cut into individual blanks of a predetermined length. More particularly, it relates to such a method for sorting paperboard blanks into individual blank groups in an orderly manner when a cut length change is effected, so that the blanks formed after the size change do not interfere with blanks formed before the size change. The present invention is especially concerned with a novel technical means capable of controlling the conveyor speed so that the second and subsequent blanks formed after the size change may accumulate upon the previous blanks in an orderly shingling manner.
2. Description of the Prior Art
In a corrugator line for continuously producing a web of corrugated paperboard, the output web of corrugated paperboard is cut into individual blanks of a predetermined length by means of rotary cutters generally during the final production step. The individual blanks are then fed into various types of processing machines such as, for example slotters and creasers. At this time, the individual blanks of a predetermined length are supplied to a downstream stacker where the blanks are accumulated until a predetermined number of blanks is in fact accumulated. To this end, the multiplicity of blanks cut by means of the cutters are sorted into a group of blanks of a desired length and are intermittently fed upon a well-known conveyor. A representative conveyor is disclosed in FIG. 1 of Japanese Laid-Open Patent Publication No. 52-129161 and as shown therein, each of the preceding blanks of a predetermined length located upon a belt conveyor is partially overlapped by the succeeding blank of an identical size (this condition is called "shingling"). A mass of the blanks thus formed is fed by means of the conveyor in a "shingling" condition.
When a continuous paperboard web is cut into blanks, the cut length has to be frequently changed in order to suit the various requirements specified by customers. Thus, when the change in cut length (hereinafter referred to as the "order change") is achieved, the rotational speed of the rotary cutters in the corrugator line is changed, so that individual blanks of a desired length may be cut thereafter.
As should be apparent, when an order change is made, blank groups comprising blanks having a different cut length in accordance with a new length order are fed following the blank groups comprising blanks having a cut length in accordance with the old length order. It is, therefore, necessary to positively isolate and sort the two different groups of blanks, one for the old order and the other for the new order, so that during transportation upon the conveyor, the leading end of the first blank for the new order does not interfere with the trailing end of the last blank for the old order.
Various methods have been heretofore proposed in order to sort paperboard blanks being conveyed upon a conveyor. Basically, three kinds of blank sorting methods have been heretofore proposed depending upon the various arrangements of the conveyor. For instance, the first method is disclosed within Japanese Laid-Open Patent Publication No. 60-258055 filed by the same assignee as the present invention. As shown in FIG. 1, the sorting apparatus for carrying out the first method includes sandwich conveyors 16,18 and first and second conveyors 22,24 arranged in series downstream of cutters 10 arranged within the vicinity of the final production step of a corrugated blank production line. The first conveyor 22 includes a vacuum box 34 mounted for an underside thereof, which vacuum box applies a vacuum suction force to blanks 14 conveyed onto the conveyor 22. As noted hereinabove, the blank groups 14 cut from a continuous paperboard web 12 include two different length blanks, one having a first desired length formed in accordance with an old order and the other having a second desired length formed in accordance with a new order. Since this distinction is important in describing the present invention, the blank formed in accordance with the old order will be designated by the symbol "OS"; and the blank formed in accordance with the new order will be designated by the symbol "NS".
When an order change is effected, the last sheet OSn of the blanks OS cut before the size change, which has been cut by means of the cutters 10 and fed onto the sandwich conveyors 16,18, is sensed by means of a suitable sensor when it has arrived at the first conveyor 22. Thereupon, as shown in FIG. 2(a), the first and second conveyors 22,24, which have been driven at a regular speed Vc, are simultaneously accelerated to a speed Vr which is faster than the regular speed Vc(Vc-Vr), thereby transferring downstream, at a relatively high rate of speed, the blank groups OS which were formed before the effected size change. When all the blanks OS have been transferred to the second conveyor 24, as shown in FIG. 2(c), the sensor detects this state so as to stop the first conveyor 22 (Vr.fwdarw.Vo), so that the blanks NS formed after the size change may be accumulated upon the first conveyor 22 in a shingled condition. As soon as this occurs, the first and second conveyors 22,24 are returned to the regular speed Vc (Vo.fwdarw.Vr and Vr.fwdarw.Vc, respectively).
According to this method, when the first blank NS-1 formed after the size change arrives at the first conveyor 22, vacuum suction force produced within the conveyor 22 will effectively act upon the blank NS-1 so that the latter may be properly held in place upon the conveyor 22, as shown in FIG. 2(c). However, since the previous blank NS-1 covers the vacuum suction area of the conveyor 22, the vacuum suction force will not effectively act upon the second and subsequent blanks NS-2, NS-3 . . . , as shown in FIG. 2(d).
Normally, the operating speed Vb of the sandwich conveyor 18 is 20% faster than the maximum production speed Vdf max of the corrugated blank production line. Practically, the fastest machine operates at 300 m/min. Thus, the second and subsequent blanks NS-2 . . . are fed by means of the sandwich conveyor 18 at a high rate of speed Vb and are discharged to the first conveyor 22, and at the moment of discharge, the blanks NS-2 . . . have considerable inertia resulting from the high rate of speed Vb. In addition, as explained above, the vacuum suction force is not acting upon the second and subsequent blanks NS-2 . . . ; they are only subjected to pressure exerted by means of brushes (which will be described later), or they are frictionally braked as they engaged the surface of the preceding blank NS upon the first conveyor 22.
For this reason, the second and subsequent blanks NS-2 . . . discharged at the speed Vb tend to slide upon the previous blank NS and overrun the same in the forward direction, or move sideways due to uneven frictional resistance. In an extreme case, the forward end of the succeeding blank may strike against the rear end of the preceding blank (this condition is called "billiards phenomenon") causing undesired problems to the blanks, such as folds and bends. All of these causes have led to unusual conditions wherein the blanks are not fed upon the conveyor in an orderly shingled manner.
The second method is disclosed within Japanese Laid-Open Patent Publication No. 52-129161 and as shown therein, the apparatus for performing the method includes a liftable stopper for stopping blanks which is disposed between belts of a conveyor. The stopper may be lifted at predetermined times so as to thereby forcibly stop the blank specified in accordance with a new order, the succeeding blanks being accumulated upon the stopped blank. Here again, the second method entails the same problem as the first method.
The third method is to provide a variable speed motor as the power source for the first conveyor which is entirely separate from the power source for the second conveyor. When blanks formed after the size change are arriving at the first conveyor, the first conveyor is operated at a lower speed than the speed Vc during normal operation. This method, however, requires an expensive, variable speed motor. Also, the speed ratio of the motor relative to the sandwich conveyor tends to become large, causing the overlapping condition to deteriorate with time.